The present invention relates in general to the field of robotics, and in particular to a new and useful robotic wrist joint which has a spherical member which can be positioned at any orientation within its work space without encountering a singularity point.
Known mechanical robot wrist joints can be classified into three types. These are the pitch-yaw-roll type the roll-bend-roll type and the three roll type. "Three Roll" is a trademark of the Cincinnati Milacrom Company.
All three types either get "twisted" or have other singularity problems associated with them. Singularities are points where a high acceleration or velocity is needed to maintain motion constraints.
See U.S. Pat. Nos. 4,296,681; 4,068,536; and 4,045,958.
Also see U.S. Pat. No. 4,194,437 to Rosheim which shows a hydraulic servo mechanism having at least two degrees of freedom and utilizing a spherical member which is driven by hydraulics.
U.S. Pat. No. 4,073,201 to Taylor et al is relevant for its showing of a powered wrist joint which can be used on a robot arm and which uses a C-shaped configuration.
U.S. Pat. No. 3,973,469 to Humen is relevant for its showing of a power transmission mechanism which utilizes a spherical member and prbvides several degrees of freedom. Also see U.S. Pat. No. 2,681,783 to Smith which shows a selflevelling device utilizing a sphere and socket arrangement for providing several degrees of freedom.
In developing the present invention, a study was made of the various wrist joint designs.
The pitch-yaw-roll configuration was noted to have a singularity point located at a position where the revolute joints were near their limits. The singularity itself came about as the wrist attempted to "untwist itself" in order to achieve desired trajectory and velocity constraints. The resulting motion involved extremely high velocities and acceleration sometimes resulting in shut-down of the system as the inertial forces deflected the joints past their limit stops. Tests were conducted using the Cincinnati Milacrom T3 robot. It was determined that a problem in the pitch-yaw-roll design was the distance between the axes of rotation. It was noted that as the wrist became larger, higher velocities and accelerations would have to be achieved in order to maintain motion within the given constraints.
The roll-bend-roll wrist configuration was then studied and it was observed that a singularity point occurred as one attempted to bend the wrist around an axis perpendicular to a second joint of the wrist. An attempt to accomplish this motion resulted in high velocity roll of the first wrist joint. A singularity problem was also encountered when the robot was moved in a "world coordinate mode". A PUMA 600 was used for these tests. It was found that as the wrist was rotated past its singularity point, the first roll joint in the wrist would rotate at a high velocity and acceleration. Many times the joint would reach its limit stops before completing the required motion. The singularity problem was found to be extremely prevalent in this robot design.
It was determined that in order to avoid some of the singularity problems, the distance between axes of rotation should be reduced and also physical limitations should be avoided which caused the singularity problems. As will be described more fully later, the desire to minimize the distance between the axes of rotation led to the use of a spherical joint. A spherical joint with all axes of rotation intersecting at a single point was found most desirable.